1. Field of the Invention
The present invention relates to an optical recording medium for high density writing (recording) and reading (reproduction) using short wavelength laser light. More particularly, the invention relates to a write once optical recording medium complying with the DVD standards which is capable of high density recording and reproduction of information with small spot light by the use of a highly sensitive, highly reliable, organic dye thin film changing in reflectance with the wavelength of laser light.
2. Description of Related Art
Recordable optical information recording media are in very widespread use. An optical recording medium is characterized in that it neither wears nor degrades, since it is free from contact with a recording/reproducing head. Using a light beam with a more tiny spot diameter, the optical recording medium has attracted wider attention as a large capacity information carrier, and its energetic development is under way. Such an optical information recording medium acts, for example, by focusing a laser light beam on a recording layer to convert it into a thermal energy, changing the properties of a recording film by fusion, decomposition, removal or the like to make recording (form pits), and reproducing recorded information by utilizing differences between the recorded area and the unrecorded area in terms of the quantity of reflected light.
The recording film initially put to practical use was a chalcogenide type metallic film typified by a tellurium-based one. However, this type of film was harmful to the human body, and was formed by a dry process, requiring a high manufacturing cost. Furthermore, the need for a higher density of recording has increased. Thus, there have been increases in proposals and reports on recording media using a recording layer essentially consisting of an organic dye, instead of a recording layer comprising an inorganic metal thin film typified by a tellurium-based one. The reason behind this is that the recording layer essentially consisting of an organic dye is lower in reflectance than a metallic recording layer, but possesses the following satisfactory properties:
(1) Can be formed as a thin film by a low cost wet process typified by spin coating.
(2) Is highly resistant to oxidation and free from corrosion.
(3) Has so low heat conductivity compared with metals that, unlike a metallic recording film, heat does not affect the surroundings, and local heating is possible.
(4) Can form recording pits of a clear shape, ensuring high sensitivity.
As to a structure, many optical information recording media have been proposed which have a so-called air sandwich structure in frequent use comprising an air layer on a recording layer composed of a dye film, or a structure capable of obtaining a reproduction signal in compliance with the CD standards (Japanese Patent Application Publication No. 759343/1991, Japanese Patent Application Laying-open No. 87341/1990, Japanese Patent Application Laying-open No. 67352/1993, and Nikkei Electronics, Jan. 23, 1989 Issue, No. 469, page 107).
A known example of a medium structure complying with the CD standards comprises a light absorbing layer comprising an organic dye formed on an optically transparent resin substrate, a light reflecting layer of Au or the like formed on the light absorbing layer directly or via a rigid layer, and a resin protecting layer formed on the light reflecting layer. That is, an organic dye film alone is not enough to give a high reflectance of 65% or more. Thus, the light absorbing layer comprising the organic dye is formed on the optically transparent resin substrate, the light reflecting layer of Au or the like is formed on the light absorbing layer directly or via the rigid layer, and the resin protecting layer is formed on the light reflecting layer to prepare the optical recording disk complying with the CD standards.
According to this optical recording disk, the organic dye layer absorbs light when irradiated with laser light, whereupon it becomes fused or decomposed, and softens the substrate. As a result, the dye material and the substrate mingle at the interface. Finally, the interface deforms to form record pits. In the resulting deformed recording layer pit area, reflectance varies according to an optical phase difference as with a CD, whereby recorded information can be read. Known as examples of the organic dye used are squarylium dyes (Japanese Patent Application Laying-open Nos. 46221/1981, 218398/1988, 178494/1989, 139047/1993 and 44904/1995), naphthoquinone dyes (Japanese Patent Application Laying-open Nos. 290092/1986, 432/1987, 168201/1988 and 139047/1993), azo dyes (Japanese Patent Application Laying-open Nos. 161069/1995, 251567/1995 and 99467/1996), phthalocyanine dyes (Japanese Patent Application Laying-open Nos. 82094/1982, 82095/1982, 156550/1995, 16068/1995 and 52544/1995), and cyanine dyes (Japanese Patent Application Laying-open Nos. 24692/1984, 87341/1990, 320869/1994, 338059/1994, 199045/1994, 262611/1995 and 201288/1987, and Japanese Patent Application Publication No. 4981/1995 and Japanese Patent No. 2547033) of the following general formula (II): ##STR1## where S.sup.1 and S.sup.2 each represent an alkyl group, an aryl group or an alkoxy group, W.sup.1 and W.sup.2 each represent a substituent such as a halogen atom, a hydrogen atom, an alkyl group, an alkoxy group, an aryl group, an alkoxysulfonyl group, a sulfonylalkyl group, or a cyano group, and Z.sup.1 and Z.sup.2 each represent a substituent such as a sulfur atom, an oxygen atom, a selenium atom, or an ethylene group.
Of the dyes exemplified above, the cyanine dyes are mainly used, because they can give a high S/N ratio, etc. with high sensitivity, and they have satisfactory thermal characteristics and enable thin film formation. In particular, the cyanine dyes are used generally frequently which have absorption and reflectance generally in the range from the wavelength of a semiconductor laser to around 780 nm to 830 nm, and in which the number q of the methylene chains at the middle of the molecular structure of the general formula (II) denotes 2, because these dyes comply with the CD standards. However, a recording film containing such a dye has the fatal drawback of being unable to serve for a short wavelength laser that the present invention targets.
This recording film containing the dye is also believed to be problematical in terms of degraded reproduction, long-term stability of the dye film, C/N ratio, and jitter component. The causes of these problems may be (i) the phenomenon that the dye itself deteriorates and discolors because of heat accumulation in the recording film area caused by long-term exposure to reading light, (ii) the phenomenon that reading light is absorbed at the interface of the recording film where the pit areas and the unrecorded areas should be discriminated, whereupon the fusion or thermal deformation of the interface gradually occurs, and that similar heat accumulation breaks the boundaries of the pits, (iii) oxidative degradation (discoloration) of the dye due to single state oxygen generated by energy transfer from the dye during optical pumping to oxygen present in the atmosphere, and (iv) degradation phenomena during long-term storage, such as changes in transmittance of the dye due to natural light, oxidation of the cyanine dye, etc., and changes in transmittance and noise generation ascribed to the association or aggregation of the dye due to oxygen or moisture. Varieties of proposals have been put forth in attempts to solve these problematical phenomena (e.g., Japanese Patent Application Laying-open Nos. 201288/1987, 201289/1987, 66541/1982, 124894/1984, 203247/1984, 11090/1982, 44389/1985, 71296/1985, 1594/1988, 11090/1982, 38879/1993 and 262611/1995, and Japanese Patent Application Publication No. 4981/1995). However, sufficient solutions have not been achieved.
Optical recording media with a high recording density complying with the DVD-ROM standards are also under development. These media use a semiconductor laser wavelength of about 600 to 680 nm, a shorter wavelength than for the current CD, and utilize an SHG element or the like to shorten the wavelength of light, thereby restricting the quantity of light from an objective lens up to the laser diffraction limited so as to decrease the beam spot diameter for an increased recording density.
To develop a dye material for DVD-ROM, there have been attempts to improve cyanine dyes for CD typified by the compounds of the general formula (II), and proposals of cyanine dyes for short wavelengths (e.g., Japanese Patent Application Laying-open Nos. 38879/1993, 40162/1994, 320869/1994, 186530/1994, 199045/1994, 262611/1995 and 306074/1996, and Japanese Patent Application Publication Nos. 21798/1989 and 4981/1995). However, may problems still remain unsolved, such that the recording layer fails to achieve sensitivity in agreement with the desired wavelength, or film stability (reliability), and that during high density recording, thermal interference by adjacent pits increase jitter components included in reproduced signals.